High-energy solid formulations, such as propellants, explosives, gasifiers or the like, generally consist of particulate solids, such as fuel material, oxidizers, or both, held together by an elastomeric binder. These high-energy formulations may also include a liquid plasticizer, such as a nitrate ester, which contributes to the elastomeric characteristics of the binder and adds additional energy to the formulation.
While the elastomeric binder is an important means of dispersing and immobilizing the fuel material and oxidizer, the materials used in the binder generally burn with substantially lower energy than does the fuel material itself. The binder thus imposes a limit on the energy content available from the fuel material. One way to minimize this limitation is to use an elastomeric binder which releases as much energy as possible when burning with the fuel material. It is desirable, therefore, that the elastomeric binder have pendant groups which themselves are relatively high in energy.
U.S. Pat. Nos. 4,393,199, 4,483,978 and 4,988,797, the teachings of which are incorporated herein by reference, are directed to methods for carrying out cationic polymerization of cyclic ethers to form polymers that can be cross-linked to form elastomeric binders for use in high-energy formulations. Generally, cationic polymerization involves initiation using an adduct, i.e. , an initiator, which is a polyhydric alcohol (e.g., a diol, such as 1,4-butanediol) in conjunction with an acid catalyst such as boron trifluoride or an etherate of boron trifluoride. This adduct complexes with a cyclic ether monomer to form an activated cyclic ether. The activated oxetane monomer then reacts with an unactivated oxetane monomer, opening up the oxetane ring and forming a species with a hydroxyl group on one end and an activated oxetane ring on the other end. The activated oxetane ring at the end of the propagating polymer chain reacts further with another unactivated oxetane, and polymerization proceeds in this manner until substantial exhaustion of cyclic ether monomers or, until the reaction is terminated in some other manner.
In these previously used methods for carrying out cationic polymerization, boron trifluoride etherate is the acid catalyst primarily and preferably used. The use of this ether-containing catalyst results in polymers having good molecular weights and functionalities when highly reactive, cyclic ether monomers, e.g., 3,3-bis(ethoxymethyl)oxetane (BEMO), are used. However, it has been found that the use of boron trifluoride etherate results in less satisfactory polymers when cyclic ether monomers substituted with high-energy, electron-withdrawing pendant groups are used, e.g., 3,3-bis(azidomethyl)oxetane (BAMO) and 3-nitratomethyl-3-methyloxetane (NMMO). In the case of polymers and copolymers formed from cyclic ether monomers substituted with high-energy, electron-withdrawing pendant groups (e.g., BAMO and NMMO), substantial chain termination by both the diethyl ether and the fluoride ion of the boron trifluoride etherate catalyst is observed.
These undesirable side reactions reduce the hydroxyl functionality of the polymer, thereby detracting from the properties of the final polymer product. More particularly, monofunctional materials such as polyethers terminated with --CH.sub.2 F or --OCH.sub.2 CH.sub.3 groups will not form elastomeric materials when cured with polyisocyanates. Such monofunctional materials degrade the overall properties of the polymer, namely the mechanical properties. It has been found, for example, that the polymerization of 3,3-bis(azidomethyl)oxetane with boron trifluoride etherate (BF.sub.3 .cndot.Et.sub.2 O) in the presence of butanediol gives a prepolymer with GPC (Gel Permeation Chromatography) molecular weight (weight average) of 5,386, a polydispersity of 1.35 and a functionality of 1.56.
Both the reduction in hydroxyl functionality and the lack of uniformity in chain length (i.e., high polydispersity) are considered to be disadvantages with respect to forming cross-linked elastomers, e.g., by curing with polyfunctional isocyanates. Moreover, many mechanical and elastomeric properties of the cross-linked elastomers are dependent upon the length of the polymer molecules between the cross links. Thus, high-polydispersity gives rise to unpredictable mechanical and elastomeric properties of the cross-linked elastomers.
U.S. Pat. No. 4,988,797, the teachings of which are incorporated herein by reference, reported that a reduction in the concentration of the boron trifluoride etherate catalyst used could reduce such undesirable termination reactions by a "reactive oligomer" mechanism. Unfortunately, in reducing the concentration of boron trifluoride etherate used, the reaction rate of the polymerization reaction is significantly reduced and thus, this method is impractical for large-scale, commercial applications.
In view of the foregoing, there remains a need for improved methods of cationic polymerization for energetic, cyclic ethers which provide for greater control of the polymerization reaction. The present invention remedies this need by providing such methods.